The present disclosure relates generally to a fluid mass flow rate measurement apparatus based on the Coriolis principle and specifically to methods for manufacturing an improved Coriolis flow rate sensor constructed from a polymeric material.
Coriolis mass flowmeters can be used to measure the mass flow rate of a fluid flowing through a closed conduit. Traditional Coriolis flowmeters employ various configurations of one or two tubes (through which fluid flows) that are oscillated in a controlled manner allowing measurement of Coriolis induced deflections (or the effects of such deflections on the tube(s)) as an indication of fluid mass flow rate flowing through the sensor.
Much of the Coriolis flowmeter prior art is concerned with using metal alloy flow tubes as the flow-sensitive elements. While the prior art also indicates the theoretical possibility that plastic may be substituted for metal in a flow tube, at the same time the prior art teaches away from the use of plastic. U.S. Pat. No. 7,127,815 (“the '815 patent”) states at col 2, lines 16-25 that “[t]he mere substitution of a plastic flow tube for a metal flow tube will produce a structure that looks like a flowmeter. However, the structure will not function as a flowmeter to generate accurate output information over a useful range of operating conditions. The mere assertion that a flowmeter could be made out of plastic is nothing more than the abstraction that plastic can be substituted for metal. It does not teach how a plastic flowmeter can be manufactured to generate accurate information over a useful range of operating conditions.” Similar statements are found in U.S. Pat. No. 6,776,053 (“the '053 patent”) from column 1, lines 58-68 to column 2, lines 1-10.
Fundamental to the successful operation of any Coriolis flowmeter is that the flow-sensitive element (e.g., a tube in the '815 and '053 patents) must be fixedly attached to a supporting base in such a manner that a fixed, stable, and unchanging boundary condition is established for the ends of the vibrating flow-sensitive element. The '815 and '053 patents describe methods of fabricating a Coriolis flowmeter with at least one polymer (e.g., PFA (poly (perfluoroalkoxy))) tube attached to a metal support using a cyanoacrylate adhesive. Thus, the stability of the joint and the quality of the boundary condition are limited by the adhesive, which is the interface most susceptible to degradation during operation.
Another aspect of the adhesive joint described in the '815 and '053 patents is that the integrity of the coupling of the tube to the metal base is not necessarily unyielding and unchanging because of the use of the adhesive. Rather, the coupling could deteriorate over time from continuous vibration of the tube causing the adhesive joint to crack or otherwise degrade. Additionally, differential thermal expansion amongst the various materials of construction (e.g., the PFA tube, the cyanoacrylate adhesive, and the metal base) will impair the integrity of the coupling of the tube to the metal base creating an unstable boundary condition resulting in uncontrolled vibration characteristics to such an extent that performance of the device could be compromised.
The '815 and '053 patents describe properties of PFA tubing which, to facilitate binding of the cyanoacrylate adhesive to the PFA tube, is subjected to etching (a process referred to in the '815 patent) that requires submersing and gently agitating PFA tubes in a heated bath containing glycol-diether. This etching process adds cost and complexity to the fabrication of the flowmeter and may not necessarily yield tubing suitable for flowmeter fabrication on a consistent basis.
Other art that describes Coriolis flowmeters with plastic flow tubes has different problems. U.S. Pat. No. 6,450,042 (“the '042 patent”), U.S. Pat. No. 6,904,667, and U.S. Patent Application Publication No. 2002/0139199 describe methods of fabricating a Coriolis flowmeter via injection molding, and forming the flow path from a core mold made from a low-melting point fusible metal alloy containing a mixture of Bismuth, Lead, Tin, Cadmium, and Indium with a melting point of about 47 degrees Celsius. The '042 patent asserts that “ . . . with the possible exception of a driver and pick offs, and case, the entirety of the flowmeter is formed by injection molding.” U.S. Pat. No. 6,450,042, column 2, lines 65-67. This method of fabrication presents significant problems and limitations. During the injection molding process, hot plastic is injected into a mold at temperatures that can exceed 350 degrees Celsius at pressures exceeding 5000 psi. When fabricating thin-wall or small diameter flow passageways (e.g., 2 mm to 4 mm internal diameter with wall thickness <2 mm) such melt temperatures and pressures will likely damage the comparatively narrow (and flexible) fusible metal core (e.g., possibly melting its surface) resulting in deformation and contamination of the flow passageways to such an extent that the device could be rendered unusable.
Furthermore, in semiconductor, pharmaceutical, bio-pharmaceutical, or other critical high-purity process applications, it is important to avoid metallic contamination, however infinitesimal. Unlike a solid core (e.g., stainless steel), the comparatively soft fusible core described in these references can partially melt or abrade during the injection molding process, allowing metal atoms to mix and become embedded within the injected plastic, thus permanently contaminating the flow passageway rendering the device unsuitable for high-purity semiconductor manufacturing applications.
Furthermore, in plastic injection molding processes, it can be desirable that various molded features have similar thicknesses because otherwise the molded part may not form properly (due to volume changes of the part during cooling). With reference to the '042 patent, this means that all structural features of the Coriolis flowmeters (e.g., wall thickness of the flow-sensitive elements, isolation plates (or “brace bars”), inlet and outlet flanges, manifold walls) all have similar thicknesses. But consequences of forming the entirety of the flowmeter by injection molding are structural and/or dynamic design limitations or compromises that could adversely limit flowmeter performance.
U.S. Pat. No. 8,404,076 and U.S. Patent Application Publication No. 2013/0174670 both describe inventions including a structure employing flow-sensitive elements fabricated from a polymeric material in which the flow passageways are formed out of a single piece of elastic polymer material. The flow passageways are fabricated by machining (e.g., drilling) them in the single polymer piece from an exterior surface after attachment of the single polymer piece to a manifold. After drilling, the external holes from the drilling are sealed. Alternatively, the structure can be fabricated by injection molding, the flow passageways being formed by a combination of a solid core employed within the mold and/or secondary drilling operations after the part is removed from its mold.
One disadvantage of this is method is that, due to fabrication limitations, forming the flow passageways with solid cores within a mold and/or drilling may necessarily require larger than desired wall thickness of the flow passageways (e.g., >1 mm). Larger wall thicknesses can limit a device's flexibility and hence measurement sensitivity at low flow rates. Another disadvantage is that the presence of corners at the intersection of adjacent linear-segments (e.g., square “U” or triangular shapes) can become sites for the accumulation of solids when measuring slurries. This accumulation will cause increased pressure loss compared to that of a curvilinear structure not having sharp bends or discontinuities in the flow path.